Speaker systems with integrated amplification electronics provide simple, compact audio transducers for multimedia personal computers. These speaker systems, referred to herein as multimedia computer amplified speaker systems, typically include pairs of wide-band speaker drivers mounted in separate housings with amplification electronics incorporated into one or both housings. Because compactness is desirable, small, wide-band speaker drivers (e.g., 3-inch diameter cone speaker drivers) are commonly used. Speaker systems of this type can offer acceptable stereo sound quality for some applications, but discerning listeners and demanding applications can require audio performance beyond the capabilities of such systems.
Bass sounds (i.e., sounds of frequencies less than about 150 Hz) are the most difficult for compact wide-band speaker systems to reproduce. The production of satisfactory bass levels is problematic for two reasons. Small speaker drivers have poor bass response because of their small physical size. In addition, the human ear is relatively less sensitive to bass frequencies than higher treble frequencies, particularly at low sound levels or volumes. The transduction of treble frequencies, which are designated herein as frequencies greater than about 150 Hz, does not suffer these difficulties.
In order to provide full, rich bass sound from a relatively small driver in a small enclosure, as in computer multi-media speaker applications, the electronics of the driving circuit are equalized to add a bass boost that compensates for limitations of driver size and enclosure volume. This bass boost can introduce artifacts into the audio signal that is delivered to the driver. For example, with most of the power provided by amplifier circuits typically being directed to bass frequencies, there is typically little capacity for increased volume levels. As a consequence, increasing the volume level will frequently result in amplifier clipping or sound distortion from driver over-excursion.
Past solutions to this problem have been to dynamically adjust the amount of bass boost relative to the desired loudness. As the volume is turned up, the bass boost, or relative bass level, is turned down. This allows the apparent loudness of the speaker system to increase without increasing undesired distortion levels. Typical circuit topologies to provide these dynamic adjustments generally fall into two classes: amplitude controlled variable resistance circuits and amplitude compressors/soft clippers. Both circuit types suffer from disadvantages.
Amplitude controlled variable resistance circuits suffer from associated attack and decay time delays during which bass boost adjustments are incomplete, resulting in discernible audio artifacts until the adjustments are completed. In addition, these types of circuits can introduce discernible distortion into the resulting audio signal. Amplitude compressors/soft clippers provide no control over frequency response. The general frequency response is largely unaffected by the compressing action. The fundamental frequencies are passed through to the amplifier, and the low frequencies can cause over-excursion in the driver.
Examples of prior dynamic equalization systems are illustrated by U.S. Pat. No. 5,359,665 of Werrbach and U.S. Pat. No. 5,361,381 of Short. Both of these systems employ control signals that are generated by an active circuit component to control signal compression or equalization. The generation of such control signals is delayed with respect to the signal being controlled, thereby introducing attack and decay time delays during signal compression or equalization. These attack and decay time delays result in discernible audio artifacts until the adjustments are completed.
Accordingly, a dynamic bass equalization circuit of the present invention has an amplitude dependent gain that is dependent upon the audio electrical signal amplitude and a dynamically adjusted frequency response that varies with the amplitude dependent gain. In one implementation, the dynamic bass equalization circuit includes a Sallen-Key high pass filter that includes an amplifier with a real-time negative feedback path. The dynamically adjusted frequency response is provided by a parallel pair of reversed diodes that are connected in the negative feedback path.
A dynamic bass equalization circuit according to the present invention operates without the attack or decay times that are characteristic of prior amplitude controlled variable resistance bass equalization circuits and equalization circuits employing active circuitry for generating control signals. In addition, the bass equalization circuit of this invention has amplitude dependent gain and frequency response modification that are not available in prior amplitude compressors/soft clipper bass equalization circuits. As a result, the present invention provides dynamic bass boost without the acoustic artifacts characteristic of attack or decay times or the over-excursion of speaker drivers that can occur with equalizers having fixed frequency characteristics.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.